@bking, in your original post you said ‘working perfectly…slows down’. Now you say ‘why motor don’t go at the speed’? Is this because of the changes I suggested?
In your Arduno sketch what is USBSerial? (Lots of times I’ve used normal Serial which goes via USB to the PC). What type of Arduino? What is the physical arrangement?
PC–usb-cable—Arduino----10-wires----Stepper-Controller ?
I’m studying your sketches…